Implement ext-foreign-toplevel-list

[bbb651]

Tested with: https://git.sr.ht/~leon_plickat/lswt

[billksun]

This PR may fix an issue with running COSMIC on niri: Drakulix/cosmic-ext-extra-sessions#12 (comment)

[YaLTeR]

This should be good to merge now. Is there any bar to test the dynamic updating of the list? One of the problems I just fixed was missing send_done().

[sodiboo]

We already have an implementation of wlr-foreign-toplevel-manager in src/protocols/foreign_toplevel.rs, which is conceptually a very similar protocol. Would it be better to pair these, augmenting that file to support both protocols? It's fairly straightforward to do so, sodiboo@d2d51bc still cleanly applies to the main branch. This way, the general behaviour between both protocols will be more consistent and there's only one "manager" type in the niri codebase to deal with.

[YaLTeR]

Sounds like a good idea actually